Design of wide band bow-tie slot antennas for multi-frequency operation in CMB experiments

Transcription

1 Design of wide band bow-tie slot antennas for multi-frequency operation in CMB experiments Angel Colin Abstract This report presents two proposals of antenna designs suitable to be included in arrays for multi-frequency operation in the ranges from to 6 GHz and from 3 to 3 GHz, both aimed to be applied in Cosmic Microwave Background (CMB) experiments. The antennas exhibit small sizes and wide bandwidth approaching 3% and 79% respectively. The radiation characteristics for a single element and the total directivity for configurations of 5x5 and 5x5 elements array were simulated by means of HFSS (Ansoft) software. Introduction Many new experiments for astrophysical studies based on CMB observation require multi-band measurements. As an example, the detection of clusters of remote galaxies can be performed by comparing CMB photon fluxes at 5 and 22 GHz, using the model of spectral deformation of the Planck law due to the Sunyaev-Zeldovich (SZ) effect []. Up to now, these requirements can be met by means of optical filtering, defining appropriate photometric bands, but it generally consumes a higher surface of the available focal plane. A proposal to overcome this problem had be shown using multi-frequency detector arrays composed by antenna-coupled bolometers associated with millimeter circuits and microstrip transmission lines [2]. On the other hand, the anomalous microwave emission is being studied as additional component of the diffuse galactic foregrounds. Previous studies have demonstrated its evidence emitting in the frequency range from to 7 GHz [3], and recently had been measured at frequencies of 3 GHz [4] although it can be observed up to 6 GHz but at lower emissivity levels. Angel Colin Instituto de Física de Cantabria (CSIC-UC), Av. Los Castros s/n, 395, Santander-Spain. acolin@ifca.unican.es

2 2 Angel Colin Usually the array systems that operate in different bands are designed using separate antennas for each band. With the advent of the new technologies now it turns to be possible and desirable to design a single antenna that operates in several bands for a same set-up system. Therefore a wideband antenna that covers the desired range is required. In this report we propose two designs of antennas suitable to be included in arrays for multi-frequency operation in the ranges from to 6 GHz and from 3 to 3 GHz respectively, aiming towards astrophysics applications. Both designs were based on a previous model reported by Evangelos S. et al. [5]. Each single element design exhibit a wide bandwidth of 3% and 79% with VSWR < 2. The return loss, the antenna impedance and the far field radiation patterns are presented. The total directivity for configurations of 5x5 and 5x5 elements array is also presented. The simulation results computed by means the commercial software HFSS (Ansoft) provide a good expectative for the fabrication of the prototypes. 2 Antenna design The proposed designs consists of a bow-tie slot antenna fed by a Co-Planar Wave guide (CPW) printed on Alumina/A996 with 3µm gold electroplated substrate with.254 mm of thickness, relative permittivity of, and conductor loss (tan δ) of.. The geometry and parameters of the proposed antenna are shown in Fig.. For design I, a=.5, b=, c=.22, d=.27, e=., f=.2, g=6.5, h=4.85, i=.2, j=.45, and k=.254. For design II, a=.5, b=.2, c=.72, d=.23, e=.5, f=.5, g=, h=.79, i=.2, j=.9 and k=.254. All dimensions are in mm. The substrate sizes are x2 mm 2 and 2x2 mm 2 respectively. Fig.. Schematic geometry and antenna parameters.

3 Title Suppressed Due to Excessive Length 3 3 Simulation results All parametric variables were controlled in the HFSS design environment considering open radiation type boundaries for the substrate s surrounding box, and using an excitation wave port of 5 Ohm. Fig. 2 shows the antenna impedance in terms of real and imaginary parts. It exhibits two out-of band resonances at 47 and 58 GHz for design I, whereas in design II, the resonance appears at 35 GHz. The return loss and VSWR are shown in Figs. 3 and 4. In one of them we observed that the antenna operates over a wide range that extend from 2 to 58 GHz giving a bandwidth of 3%, whereas the other extends from 3 to more than 3 GHz, for our purposes we considered 3 GHz as limit, hence we got a bandwidth of 79%, both cases exhibit a VSWR < 2. The Fig. 5 shows the computed radiation patterns at the operating band center frequencies, 35 and 25 GHz respectively. At these frequencies the antennas have beamwidths of 9 o and 5 o in the E-plane and of 45 o and 6 o in the H-plane. We observed that at frequencies of 42 GHz and 73 GHz the antennas have the best matched hence we computed its total directivity considering arrays of 5x5 and 5x5 elements giving the maximum radiation intensity (U) of 57.2 and W sr, with peak directivity of 3.8 and 3.5 respectively, their corresponding radiation patterns are shown in Fig Design I (Antenna impedance) Z [Ohm] Design II. (Antenna impedance) Z [Ohm] Fig. 2. Computed complex impedance.

4 4 Angel Colin Design I. (Return loss) Design I. (VSWR) abs(vswr) Fig. 3. Return loss and VSWR for design I. Design II. (Return loss) Design II. (VSWR) abs(vswr) Fig. 4. Return loss and VSWR for design II.

5 Title Suppressed Due to Excessive Length E - H planes at 35 GHz E - H planes at 25 GHz Fig. 5. E-H plane radiation patterns at 35 and 25 GHz. 5x5 elements array Theta [deg] x5 elements array Theta [deg] Fig. 6. Total directivity at 42 and 73 GHz.

6 6 Angel Colin 4 Conclusions This report gives a first estimation of the performances expected for wide band slot antennas operating simultaneously at different band frequencies required for the study of CMB. The concept is currently under test to validate the design tool. The simulations results provide good expectative for the fabrication of the prototypes. References. Nati, F. et al.: The OLIMPO experiment. New Astronomy Reviews. vol. 5, (27) 2. Rauly D. et al.: Design of two-band 5-22 GHz superconducting bolometric detection structure. PIERS Proceedings, Cambridge, USA (July 2-6, 28) 3. Watson, R.A. et al.: Detection of anomalous microwave emission in the perseus molecular cloud with the COSMOSOMAS experiment. ApJ, 624:L89, (25). 4. Dickinson, C.: Anomalous emission from HII regions. CMB Component Separation and the Physics of Foregrounds, Pasadena California, USA. (4-8 July 28) 5. Evangelos, S. et al.: Ultra wide-band bow-tie slot antenna fed a CPW-to-CPW transition loaded with inductively coupled slots. Microwave and op. tech. letters. vol. 48, (26).